1. Field of the Invention
The present invention relates to detecting and identifying metal targets in general and, more particularly, to a steerable three-dimensional magnetic field sensor system and method for detecting and identifying metal targets, such as unexploded ordnance (UXO), underground utilities, high metal content landmines and low metal content landmines buried in the soil (or visually obscured) based on the electromagnetic response of the target to a time-domain wide bandwidth electromagnetic spectrum.
2. Description of the Related Art
Most electromagnetic induction (EMI) metal detectors use a loop antenna to create a magnetic field in the vicinity of a metal target for the purposes of detection and identification. One of the most important functions of a magnetic field antenna is to project a strong magnetic field at the site of the target.
Typical loop antennas are formed of multiple turns of wire around a central axis. The magnetic field strength of a loop antenna is a strong function of distance from the antenna. Far from the antenna, along the axis of the loop antenna, the field strength varies approximately as 1/r3, where r is the distance from the plane of the loop to the object, Off-axis, the antenna field strength and direction tends to be a very complex function of position, with the field intensity very strong near the wires in the loop and weaker near the center of the loop.
One of the consequences of the loop antenna's complex spatial field strength is the fact that a metal target is excited with a complex magnetic field. When a buried target of unknown depth is scanned with an EMI sensor, the spatial distribution of the excitation field at the target is not known. Some target identification algorithms assume that the target is excited with a uniform field. If the field is in fact complex, the target's time or frequency response to the field is not well characterized. This may tend to complicate or confound a target identification algorithm.
In addition, with the target at the center of the loop, the loop magnetic field antenna only measures the vertical component of a target's decay response.
A metal target can be modeled by defining a magnetic polarizability tensor:
      M    _    =      (                                                      M              x                        ⁡                          (              t              )                                                0                          0                                      0                                                    M              y                        ⁡                          (              t              )                                                0                                      0                          0                                                    M              z                        ⁡                          (              t              )                                            )  where the diagonal components of the tensor are the time responses of the target to excitations in an orthogonal reference frame centered on the target. Models of this nature generally assume that the excitation field strength is uniform over the target's volume. For a loop antenna oriented directly over a target, the antenna only excites the vertical component of the target's time decay response, Mz(t). For accurate target classification, it is necessary to measure all three components of a target's magnetic polarizability tensor.
Accordingly, a need exists to develop a magnetic field sensor system that can project a strong magnetic field deeply into the ground; excite the target with a uniform magnetic field; and measure the three-dimensional components of the target's magnetic polarizability tensor. As noted above, prior art EMI metal detectors that use loop magnetic field antennas do not address all of these issues.